The present invention relates to a process and an apparatus for forming a dot image on a recording medium capable of controlling dot size, and particularly to a process and an apparatus for forming a dot image on a recording medium capable of controlling dot size which can be used for a formation of a gradational image. The recording medium used in the present invention has a characteristic in which a receding contact angle decreases when the recording medium is heated in a condition where the recording medium is in contact with a contact material such as a liquid or the like.
An offset printing method using a printing plates without water (water for moisturizing) is a typical one of methods in which a recording medium is divided into areas where it is easy for liquid to adhere thereto and area where it is hard for the liquid to adhere thereto. However, in this offset printing method, it is difficult to incorporate a process for manufacturing printing plates from original plates and a process for printing from the printing plates into a single apparatus. This makes it difficult to have a compact printing apparatus.
For example, even in a case of relatively compact offset printing apparatus, a plate making apparatus and a printing apparatus are separated.
To eliminate this fault of the offset printing method, there has been proposed a recording method and apparatus in which areas where it is easy for the liquid to adhere thereto and areas where it is hard for the liquid to adhere thereto can be formed in accordance with image information and in which the recording medium can be repeatedly used (a process for forming an image is reversible). The following are some of these.
1 Water-soluble developing method
After a charge has been applied from an external device to a hydrophobic photo-electric layer, a medium having the hydrophobic photo-electric layer is exposed so that a pattern having hydrophobic portions and hydrophilic portions is formed on the surface of the hydrophobic photo-electric layer. Then, a water soluble developing solution adheres to only the hydrophilic portions and is transferred to a paper or the like. Such methods and apparatus are disclosed in Japanese Patent Publication Nos.40-18992, 40-18993 and 44-9512 and Japanese Patent Laid Open Publication No.63-264392, etc.).
2 Method using a photo-chemical response of a photo-chromic material
In this method, an ultraviolet light is irradiated to a layer which contains a material such as a spiropyran or an azo dye so that a photo-chemical reaction occurs to make the photo-chromic material hydrophilic. Such method and apparatus are described in "Japanese Journal of Polymer Science and Technology" Vol. 37, No. 4 page 287, 1980).
3 Method using an action of an internal biasing forces
In this method, amorphous substances and crystalline substances are formed in a recording medium by a physical transformation, so that portions where it is easy for a liquid ink to adhere thereto and portions where it is hard for the liquid ink to adhere thereto are formed on the recording medium. An example of such is disclosed in Japanese Patent Laid Open Publication No.54-41902.
According to the previously described method 1, after the water-soluble ink is transferred to the paper or the like, the hydrophilic portions are removed by removing the charge so that it is possible to record other image information. That is, one original plate (photo-electric member) can be repeatedly used for printing images. However, in this method, an electrophotography process is basically used, so that a long time is required for carrying out the process involving steps of charging, exposing, developing, transferring and discharging. Therefore, it is difficult to make an apparatus compact, to reduce its cost and to make an apparatus in which it is unnecessary to maintain.
In the method 2 described above, it is possible to freely control the reversibility of the hydrophilic and hydrophobic properties by selective irradiation of ultraviolet and visible light. However, since a quantum efficiency is very small, a response time is extremely long and a recording speed is low. In addition, there is also a fault of image instability. Therefore, this method has still not put into practical use.
Furthermore, an information recording member (the recording medium) which is used in the method 3 has stability after an image is formed thereon, but there are occasions structural transformation occurs in the information recording member due to temperature changes prior to the recording. That is, the method 3 has a disadvantage in that it is difficult to maintain the image on the information recording member. In addition, when recorded information patterns is removed, a thermal pulse must apply to the information recording member and then it is, necessary to rapidly cool the information recording member. Therefore, it is difficult to perform frequent repetition of image formation.